3 weeks old?????? how on earth did you get a 3 week old puppy??!? you are never ever sopposed to let the puppy leave its mother until at least 8 weeks, if you do, it can cause serious developemental and behavioral problems.
Do not take it around other dogs until at least 9 weeks for sure. You want to expose the dog to other dogs as soon as possible, but your puppy is wayyyyyyyyyy to young. Its vision is not even fully devoloped yet.
3 week old puppy scares me...it should be with its mother and litter mates.